1. Field of the Invention
The present invention relates to a nonresonant knock sensor mounted to an internal combustion engine by a bolt such that when a knocking vibration occurs in the internal combustion engine, the vibration is converted into a voltage signal by a piezoelectric element held in an interior portion, and the voltage signal is conducted out to an external portion as an output signal.
2. Description of the Related Art
Conventional knock sensors include a metal sleeve having: a cylindrical portion; and a flange portion formed on a lower portion of the cylindrical portion, a thread being formed on an outer peripheral surface of an upper portion of the cylindrical portion. A first annular electrode plate is fitted over the cylindrical portion so as to be positioned on the flange portion with an electrically-insulating plate interposed, an annular piezoelectric element is fitted over the cylindrical portion so as to be positioned on the first annular electrode plate, a second annular electrode plate is fitted over the cylindrical portion so as to be positioned on the annular piezoelectric element, and a weight is fitted over the cylindrical portion so as to be positioned on the second annular electrode plate with an electrically-insulating plate interposed. A nut is mounted by being screwed onto the thread, and each of the members is clamped and held between the nut and the flange portion by fastening the nut. Finally, a synthetic resin case is molded around the metal sleeve, each of the members being embedded inside the synthetic resin case. (See Patent Literature 1, for example.)
The annular piezoelectric element has electrode layers formed over entire front and rear surfaces thereof, and is polarized in a thickness direction. The first and second annular electrode plates are formed so as to have a shape having an inside diameter and an outside diameter equal to those of the annular piezoelectric element, and are respectively placed in close contact with the front and rear electrode layers of the annular piezoelectric element to ensure an electrically-connected state. In addition, connecting rod portions project outward from a peripheral edge of each of the first and second annular electrode plates, and are electrically connected to a pair of terminals of a connector portion formed so as to be integrated with the synthetic resin case.
A conventional knock sensor configured in this manner is mounted to an internal combustion engine by a bolt inserted inside a cylindrical portion of a metal bush. If knocking occurs in the internal combustion engine, component parts such as the annular piezoelectric element and the weight, etc., vibrate against each other due to the knocking vibration. The vibration is converted to a voltage signal by the annular piezoelectric element. The voltage signal is output to an external portion through a female coupler fitted into the connector portion.
Patent Literature 1: Japanese Patent Laid-Open No. 2002-257624 (Gazette)
In conventional knock sensors, the first and second annular electrode plates are formed so as to have a shape having an inside diameter and an outside diameter equal to those of the annular piezoelectric element, and electrode layers are formed on entire front and rear surfaces of the annular piezoelectric element contacting the first and second annular electrode plates. Thus, the electrostatic capacity is fixed at a level corresponding to the thickness of the annular piezoelectric element and the surface area of the electrode layers, and the level of the output signal elicited by the knocking vibration is also fixed at a predetermined level. The level of the output signal elicited by the knocking vibration can be changed by changing the thickness or the diameters of the annular piezoelectric element but it then becomes necessary to modify the external shape of the knock sensor.
In order to solve this problem, it is necessary to form the electrode layers on the front and rear surfaces of the annular piezoelectric element partially instead of forming them over the entire front and rear surfaces of the annular piezoelectric element contacting the first and second annular electrode plates. However, if the electrode layers are formed on the front and rear surfaces of the annular piezoelectric element partially, gaps corresponding to the thickness of the electrode layers arise between the first and second annular electrode plates and the annular piezoelectric element, giving rise to problems such as that described below.
Specifically, when the polarizing processes are applied to portions of the annular piezoelectric element where the electrode layers are formed, the polarizing action also affects portions around the electrode layers where the electrode layers are not formed. Thus, electric charge arising due to pyroelectric effects accompanying changes in the ambient temperature is discharged gradually and easily via the first and second annular electrode plates in the portions where the electrode layers are formed, but accumulates in the portions where the electrode layers are not formed. The electric charge that has accumulated in the portions of the annular piezoelectric element where the electrode layers are not formed gives rise to dielectric breakdown at voltages corresponding to the gaps existing between the first and second annular electrode plates and the annular piezoelectric element and is discharged toward the first and second annular electrode plates instantaneously. The electric charge transferred to the first and second annular electrode plates by this discharge is impressed on the annular piezoelectric element in the form of a circulating current. In this case, if an electric charge having a polarity equal to that of the electrode polarity of the annular piezoelectric element is impressed, the annular piezoelectric element extends momentarily in the direction of polarization, and an electric charge having reverse polarity occurs in an interior portion. Thus, in conventional knock sensor configurations, one problem has been that if the electrode layers are formed on the annular piezoelectric element partially, noise resulting from ambient temperature changes is superposed on the output signal.